1. Field of the Invention
This invention relates to a color temperature tunable white light source and in particular to a light source based on light emitting diode arrangements. Moreover the invention provides a method of generating white light of a selected color temperature.
2. Description of the Related Art
As is known the correlated color temperature (CCT) of a white light source is determined by comparing its hue with a theoretical, heated black-body radiator. CCT is specified in Kelvin (K) and corresponds to the temperature of the black-body radiator which radiates the same hue of white light as the light source. Today, the color temperature from a white light source is determined predominantly by the mechanism used to generate the light. For example incandescent light sources always give a relatively low color temperature around 3000K, called “warm white”. Conversely, fluorescent lights always give a higher color temperature around 7000K, called “cold white”. The choice of warm or cold white is determined when purchasing the light source or when a building design or construction is completed. In many situations, such as street lighting, warm white and cold white light are used together.
White light emitting diodes (LEDs) are known in the art and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As is known white light generating LEDs (“white LEDs”) include one or more phosphor materials, that is a photo luminescent material, which absorbs a portion of the radiation emitted by the LED and re-emits radiation of a different color (wavelength). Typically, the LED die or chip generates blue light in the visible part of the spectrum and the phosphor re-emits yellow or a combination of green and red light, green and yellow or yellow and red light. The portion of the visible blue light generated by the LED which is not absorbed by the phosphor mixes with the yellow light emitted to provide light which appears to the eye as being white in color. The CCT of a white LED is determined by the phosphor composition incorporated in the LED.
It is predicted that white LEDs could potentially replace incandescent, fluorescent and neon light sources due to their long operating lifetimes, potentially many 100,000 of hours, and their high efficiency in terms of low power consumption. Recently high brightness white LEDs have been used to replace conventional white fluorescent, mercury vapor lamps and neon lights. Like other lighting sources the CCT of a white LED is fixed and is determined by the phosphor composition used to fabricate the LED.
U.S. Pat. No. 7,014,336 disclose systems and methods of generating high-quality white light, that is white light having a substantially continuous spectrum within the photopic response (spectral transfer function) of the human eye. Since the eye's photopic response gives a measure of the limits of what the eye can see this sets the boundaries on high-quality white light having a wavelength range 400 nm (ultraviolet) to 700 nm (infrared). One system for creating white light comprises three hundred LEDs each of which has a narrow spectral width with a maximum spectral peak spanning a predetermined portion of the 400 nm to 700 nm wavelength range. By selectively controlling the intensity of each of the LEDs the color temperature (and also color) can be controlled. A further lighting fixture comprises nine LEDs having a spectral width of 25 nm spaced every 25 nm over the wavelength range. The powers of the LEDs can be adjusted to generate a range of color temperatures (and colors as well) by adjusting the relative intensities of the nine LEDs. It is also proposed to use fewer LEDs to generate white light provided each LED has an increased spectral width to maintain a substantially continuous spectrum that fills the photopic response of the eye. Another lighting fixture comprises using one or more white LEDs and providing an optical high-pass filter to change the color temperature of the white light. By providing a series of interchangeable filters this enables a single light fixture to produce white light of any temperature by specifying a series of ranges for the various filters.
The present invention arose in an endeavor to provide a white light source whose color temperature is at least in part tunable.